Processes for producing methanol are known for example from EP 0 790 226 B1. The methanol is produced in a cyclic process in which a mixture of fresh and partly reacted synthesis gas first is supplied to a water-cooled reactor and then to a gas-cooled reactor, in each of which the synthesis gas is converted to methanol on a copper-based catalyst. The methanol produced in the process is separated from the synthesis gas to be recirculated, which then is countercurrently passed through the gas-cooled reactor as coolant and preheated to a temperature of 220 to 280° C., before it is introduced into the first synthesis reactor. Novel, more active catalyst generations allow an operation of the methanol production at lower and lower temperatures, which results in a better exploitation of the thermodynamic potential of the reaction and in a higher methanol yield. In this connection, it must be prevented that a condensation of the methanol product occurs in the gas-cooled second reactor, since the reactor wall temperature at the outlet can approach the dew point of the product gas.
FR 2,533,554 A1 describes a process for producing alcohols from synthesis gas in two reactor stages, wherein from the product of the second reactor stage a liquid fraction rich in methanol and higher alcohols is separated from the gas stream by condensation. The gas stream is at least partly recirculated to the first reactor stage, in order to increase the ratio H2/CO to a value≧1.8. An intermediate condensation of methanol between the two reactor stages should be avoided, in order to reduce the water content of the mixture obtained and thereby facilitate the future dehydrogenation of the alcohols.
From the U.S. Pat. No. 4,968,722 a process for producing methanol is known, in which the methanol product is removed by absorption between two reactor stages, in order to thereby increase the product yield of the equilibrium reaction in the second reactor stage. An intermediate condensation here is also described as disadvantageous, since it requires large heat exchanging surfaces.
EP 0 483 919 A2 describes a process for methanol production, in which the conversion of the synthesis gas is effected in a plurality of series-connected fluidized-bed reactors, wherein between the reactor stages a separation of methanol formed is effected by cooling and condensation.
The document US 2007/0293590 A1 describes the performance of heterogeneously catalytic exothermal gas phase reactions for the methanol synthesis from a synthesis gas in at least two series-connected synthesis reactors. Between these reactors a partial stream of the methanol-containing product is withdrawn and methanol is condensed out by multistage heat exchange with boiler feed water and deionized water. The gaseous constituents are mixed with the warm part of the product gas from the first reactor and supplied to the second reactor.